1. Field of the Invention
The present invention relates to a process of forming mask images and, more particularly, to a process of forming mask images having high resolving power, high contrast, high heat resistance, and high durability utilizing silver halide photographic images.
2. Description of the Prior Art
It is known that images of metallic silver are obtained in a silver halide photographic emulsion layer of a photographic light-sensitive material with a silver halide emulsion layer on a support by imagewise exposing and developing the photographic light-sensitive material. However, the silver images formed in a manner such as is described above are unsuitable for uses wherein high heat resistance is required since, when the silver halide emulsion layer with the silver images therein is heated to a high temperature of from about 150.degree. to about 200.degree. C., the binder present in the emulsion layer is thermally decomposed and discolors.
One of the fields requiring heat resistant images is super microphotography. A photographic film of a 35 mm size having a photographic image of an original of 9.times.14 inches (23.times.36 cm.sup.2) size reduced to about 1/10 thereof is usually called a "microfilm" and a photographic film having a photographic image further reduced to about 1/10 (about 2.times.3 mm.sup.2) is usually called a "super microfilm". That is, a microfilm is a photographic film containing photographic images whose size is reduced to about 1/10 of the size of the original and a super microfilm is a photographic film containing photographic images whose size is reduced to about 1/100 of the size of the original. The term "super microfilm" or "super microphotographic film" is used in this specification with the above-described meaning.
The image size of a super microfilm is 2.times.3 mm.sup.2 as described above or smaller and in order to project the microfilm image on a transmission type screen by enlarging to the original size, the microfilm image must be enlarged to about 100 times the size thereof (or about 10,000 times from an area standpoint the area of the microfilm image). Therefore, where the brightness of the image projected on a screen must be 100 luxes, the brightness must be 10,000,000 luxes on the images of the super microfilm when the transmission density of the screen is 1. In practice, since there is a loss of brightness due to the projection lenses, the photographic images of a super microfilm is illuminated by light of several million additional luxes. If a silver halide emulsion layer of a super microfilm containing photographic images is continuously illuminated with such an intense light, the temperature of the silver halide emulsion layer increases to a few hundred degrees (.degree.C.) due to the heat generated by the light absorbed in the emulsion layer, which results in a decomposition of the binder present in the emulsion layer and a coloring or a darkening of the images projected on the screen. In this case, the silver image-containing areas of the emulsion layer particularly tend to absorb light and hence the temperature of these areas first increases to decompose the binder in the emulsion layer and this phenomenon spreads gradually to the surrounding areas. Even the binder at the non-image areas of the silver halide emulsion layer increases, when the binder is colored slightly, the absorption coefficient of light, which results in an acceleration of the decomposition of the binder.
Also, emulsion masks and hard masks have hitherto been used in general as photo masks for micro electronic fabrication, but emulsion masks have a low contrast at the edges of the images, a weak mechanical strength, and a low durability. Also, hard masks have a high durability but they must be prepared using a complicated process. Furthermore, in order to produce hard masks, a photoetching process is required and since the photoresist used in this case has low sensitivity, there is the disadvantage that the time required for the exposure increases.
A process described in U.S. Pat. No. 3,567,447 is known as a process of making photo masks, etc. In the process described therein, the binder portions of a photographic light-sensitive material are heated after imagewise exposure and development to form gelatin binder areas containing silver images therein on the transparent support of the photographic material and exposed areas of the transparent support without silver images, after forming a layer of a metal such as chromium over the entire surface of the photographic material, the surface thereof is treated with a metal removing solution, whereby the metal layer is removed at the above-described image-containing areas while the metal layer remains as it is at the unexposed areas of the transparent support, and then the surface is treated with an aqueous nitric acid solution, whereby the silver image-containing gelatin binder areas described above are removed to provide a photomask.
However, since in the process as described above, a metal layer such as a chromium layer is formed after forming silver image-containing gelatin binder areas by heating, it has been found that the above-described technique has the following disadvantages. That is, the means and the conditions for forming the above-described metal layer are restricted (for example, if the surface of the support is treated with a strong acid or a strong alkali for improving the adhesion of the surface and the metal layer, the binder is dissolved, which makes it difficult to treat the surface using a strong acid or strong alkali and further it is unsuitable to treat the exposed surface of the support with an abrasive powder since in such case the binder is also scraped off), the treatment procedure is complicated and unwieldly, there is a possibility of reducing the durability of the photomask obtained since the surface of the ultimately obtained metallic mask images is also etched by the above-described metal removing solution and is chemically denatured (for example, when aluminum is used as the metal and sodium hypochlorite is used for removing aluminum on the silver image portions, aluminum at the non-image areas is also removed since sodium hypochloride is an etchant for aluminum), and also the edges of the photomask images obtained are not smooth or sharp.
Thus, various intense investigations for resolving these problems as mentioned above were made and as the result thereof it was found that in the cases of imagewise exposing a photographic light-sensitive material having a mask layer and a silver halide emulsion layer formed on a transparent support followed by development and fixing, heating the binder layer of the photographic material to a high temperature to thermally decompose the binder, and selectively removing the binder on the non-image areas with a solution for removing the thermally decomposed binder (for example, an aqueous solution of sodium hypochlorite), the binder at the silver image-containing areas remained together with the silver images and was not removed and further the remaining silver image areas possess a so-called resist property, that is non-permeability to an etching solution applied to the uncovered areas of the mask layer and of substantially not being swollen by the etching solution.
Furthermore, it was also found that in the cases of imagewise exposing the photographic light-sensitive material as described above followed by development and subjecting such to a silver removing bleach treatment to form silver halide images, heating the binder layer of the photographic material to a high temperature to thermally decompose the binder, and selectively removing the binder at the non-image areas with a solution for removing the thermally decomposed binder, the binder at the silver halide image-containing portions remained together with the silver halide images without being removed and further the remaining silver halide image areas had a resist property to the etching solution applied to the uncovered portions of the mask layer.
These discoveries provided the basis for U.S. patent application Ser. No. 666,996 filed Mar. 15, 1976. That is, as described therein, a photographic light-sensitive material having a mask layer and a silver halide emulsion layer formed on a transparent support is imagewise exposed, developed, and fixed to form silver images, the photographic material is heated to temperatures higher than about 250.degree. C., preferably higher than about 300.degree. C., binder is removed at the non-image areas with a binder removing solution to uncover the mask layer in the areas where the gelatin is removed, and then the uncovered mask layer is removed with an etching solution. By this process, a so-called nega-posi type image composed of the mask layer remaining at the image-exposed portions is formed.
Where a posi-posi type image is desired, that is, to leave the mask layer at the non-image areas in the above-described process, a so-called reversal development is carried out. That is, after imagewise exposing and developing the photographic light-sensitive material having the above-described layer structure without applying a fixing processing thereto, the silver images obtained are removed with a bleach solution, the silver halide remaining at the non-image areas is reduced to silver, and then the photographic material is heated and subjected to the subsequent treatments as described for the above process. An aqueous solution containing an alkali metal dichromate and sulfuric acid is used as the bleach solution in the reversal development.
However, it has now been found that when the silver images are bleached with a bleaching solution containing bichromate ions, the binder is hardened at the silver image areas to cause the following difficulties. That is, it has been found that when the heating treatment, which is the step after bleach treatment, is performed at a sufficiently high temperature above about 300.degree. C., and then the photographic material is treated with a binder removing solution, binder is selectively removed at the non-silver image areas to provide a very desirable result, while when the heating treatment is carried out at a low temperature below about 300.degree. C., the removal speed of binder at the non-image areas decreases since the binder is hardened. Thus, it is difficult under such heating conditions to selectively dissolve off with a binder removing solution the non-silver image areas formed after the removal of silver images by bleaching.